The present invention is generally related to mass media drive controllers, including HDD drives, and more particularly to an ultra-fast voltage drive for a write drive control circuit controlling the head of the drive.
The coil current waveform characteristics that are important in a HDD voltage write driver circuit are depicted at 10 in FIG. 1. In particular, the current rise time, overshoot, undershoot, and settling time are of particular interest. Desired characteristics for the coil current waveform are a fast rise time and settling time, a controllable amount of overshoot, and very little undershoot. By achieving these characteristics, a hard disk drive storage capacity can be improved.
A conventional voltage source based write coil driver circuit is shown at 20 in FIG. 2. A coil is modeled as LS, and is connected to four transistors via coil node HX and node HY. The connection of the four transistors to the coil is known conventionally as an H-bridge. The coil is typically referred to as the head. The H-bridge controls the direction of the coil current Iw, known as a coil current reversal, to responsively create magnetic flux changes through the coil.
One limitation of this conventional voltage source based write coil driver 20 is the differential transient voltage generated at nodes HX and HY conventionally known as (TRTF), driving a current reversal. This limitation can be appreciated in conventional circuit 20 shown in FIG. 2, whereby the voltage of one node can be momentarily pulled down, such as node HX, to near the second voltage rail Vee using transistor Q1. However, and disadvantageously, there is no circuit to pull up the other node, such as node HY. Thus, the voltage at each node can go no higher than established that are expressed by the following equation:
Vnode=Vccxe2x88x92(Vbe+RSxc3x97Iw). 
The coil current Iw has a nominal current of about 40 milliamps with control signals QL or {overscore (QL)}. However, during current reversal of the coil, transistors Q1 or Q2 are temporarily turned on to xe2x80x9cboostxe2x80x9d normal current and decrease the differential voltage TRTF. It can be appreciated that transistors Q1 and Q2 bypass half of the matched impedance resistors RS during this xe2x80x9cboost,xe2x80x9d however, the other series resistor RS is still in the current path. Thus, the above equation represents the maximum voltage that one node can attain when the other node is pulled down close to Vee.
There is desired an improved circuit that improves the speed of a rail-to-rail drive circuit, and which includes drive circuit providing a rail-to-rail drive voltage to large PMOS FETs pulling-up a voltage at nodes HX and HY.
The present invention achieves technical advantages as a rail-to-rail ultra-fast voltage drive circuit providing a rail-to-rail drive voltage to a subsequent drive circuit, such as a HDD write driver control circuit. The voltage drive circuit has a pair of serially connected output transistors driven by a control circuit such that an output voltage defined at an output node between the two output transistors is driven fast rail-to-rail.
This control circuit preferably comprises three serially connected FETs coupled between the voltage rails, whereby the middle FET controls the voltage at the output node. The other two FETs are coupled to a respective voltage rail, and each have their gates connected to the opposing rail. This gate xying arrangement reduces the number and size of the devices required in this control circuit, and further speeds up the rail-to-rail voltage swings of the drive circuit.